This invention relates to the art of allocating available data rate to users of a wireless communication system, and in particular, to allocating available data rate to users of fixed wireless loop, or so-called xe2x80x9cwireless local loopxe2x80x9d systems.
Typical prior art wireless systems employ a fixed data rate allocation per user. Once a user is assigned a modulation scheme, i.e., a constellation for mapping the user""s bits into symbols, the user""s data rate is fixed unless the user is assigned further, e.g., an additional one or more, time slots. Such systems are unable to take advantage of improvements in channel quality, and suffer in the event of channel quality degradation.
In accordance with the principles of the invention, the constellation mapping scheme employed may be changed on a per-time-slot basis, i.e., from time slot to time slot, so that the constellation used to encode the symbols of each time slot may be different for each time slot within a single frame and may be different for a particular time slot in different consecutive frames. In other words, several constellation mapping schemes are available, with each providing the ability to transmit a different number of bits per symbol, and the particular constellation mapping scheme employed for any time slot need be selected for that time slot only. The ability to use any particular constellation mapping scheme is dependent on the current channel quality.
In accordance with an aspect of the invention, the particular constellation mapping used for the user data of the time slot may be indicated in the preamble of the time slot. To this end, each time slot may have its own preamble that is mapped with a constellation mapping scheme which is a) known a priori, b) may be the same for all time slots, and c) may be different from the constellation mapping scheme used to encode user data in the time slot.
In accordance with another aspect of the invention, a receiver can determine the constellation mapping used for each time slot from the preamble of the time slot.
Advantageously, by having the ability to change the constellation mapping scheme employed on a per-time-slot basis the user""s data rate may be rapidly changed, i.e., increased or decreased, to correspond to the data rate that has the highest throughput under current channel conditions and the user""s specified QoS. Since the constellation mapping scheme that may be employed is a function of the channel quality, which may change over time, it is necessary to monitor the channel quality to determine which constellation mapping is appropriate to use for each time slot.